The present invention relates to an add/drop multiplexer for use in a wavelength division multiplexed optical network, and to a branching unit comprising such an add/drop multiplexer.
Wavelength division multiplexing, termed WDM, (discussed in, for example, Spirit and O""Mahony, xe2x80x9cHigh Capacity Optical Transmission Explainedxe2x80x9d, John Wiley and Sons, Chichester, 1995, and Hill, British Telecom Technology Journal, 6 (3):24-31) is a technique of considerable benefit in optimising transmission of signals through fiber optic networks. In wavelength division multiplexing, traffic signals to be sent out by a station of the network are modulated on to a number of carrier signals at different predetermined carrier wavelengths. Each predetermined carrier wavelength is allocated according to the identities of the send station and of the intended receive station. Predetermined carrier wavelengths are spaced sufficiently far apart in wavelength that they can be discriminated from each other by components of the fiber optic system, but in many networks will also need to be grouped sufficiently closely that different (often all) carrier wavelengths can be amplified satisfactorily by the same amplifier in a repeater. The carrying capacity of a single fiber is enhanced by WDMxe2x80x94rather than carrying a single signal, the fiber is simultaneously carrying several signals, each at a different carrier wavelength.
Most such transmission networks have a number of nodes at which one or more branches separate from a main trunk or ring. Typically, at these nodes one or more carrier wavelengths are dropped down one or more fibers of the branch and one or more carrier wavelengths (which may be the same as, or different from, those dropped from the trunk or ring) are added to the trunk or ring from another fiber of the branch. The component which performs such a function is an add/drop multiplexer (ADM).
WDM is particularly well adapted to efficient routing of signals between send and receive stations. As different signals have different carrier wavelengths, optical components can be used to route signals appropriately by directing them according to the carrier wavelength of the signal. Example of an ADM of this type is disclosed in Giles and Mizrahi IOOC 95, ThC2-1 pp 66-67. This add/drop multiplexer comprises two three port circulators (107,108) with a fiber Bragg grating (105) between the second ports of the circulators (FIG. 3). The input of a first fiber (101) is connected to the first port of first circulator (107) and the output of the first fiber (102) is connected to the third port of the second circulator (108). A second fiber input (104) is connected to the first port of the second circulator (108) and a second output fiber (103) is connected to the third port of the first circulator (107). The two second ports are connected to each other with the fiber Bragg grating (105) therebetween. The reflection wavelength of the fiber Bragg grating is the wavelength to be added or dropped. Signals at any other wavelength enter the first port of the first circulator (107), pass through to the second port of the first circulator (107) and through the grating into the second port of the second circulator (108), then out of the first output (102). Signals at the fiber Bragg grating reflection wavelength arriving on the first fiber pass through from the first port to second port of the first circulator (107), are then reflected by the fiber Bragg grating (105) back to the second port of the first circulator (107), where they are circulated to the third port of the first circulator (107) and to the second output (103). By contrast, signals added at this wavelength on the second fiber input (104) pass from first to second port of the second circulator (108), whereafter they are reflected by the fiber Bragg grating (105) back to the second port of the second circulator (108), and are circulated out to the third port of the second circulator (108) and out onto the first (102) output.
In wavelength division multiplexed systems of significant length, it will be necessary to amplify the traffic signals. A particularly appropriate point to amplify the traffic signals is at a branching unit, because the routing of traffic signals through the add/drop multiplexer will be accompanied by loss of power. Chawki et al, xe2x80x9cEvaluation of an Optical Boosted Add/Drop Multiplexer OBADM including circulators and fiber grating filtersxe2x80x9d, Proc. 21st Eur. Conf. on Opt. Comm. (ECOCxe2x80x9c95xe2x80x94Brussels), discloses an add/drop multiplexer of this type incorporating a bidirectional amplifier. The bidirectional amplifier is located between the first three port circulator and the fiber Bragg grating. With the configuration described above, the bidirectional amplifier will amplify all signals remaining on the trunk (from first inp;ut to first output) together with the signal dropped from the trunk to the branch (second output). The add signal (first input) will, however, not be amplified. In an alternative arrangement, instead of the fiber Bragg grating reflecting the add/drop wavelength or wavelengths, fiber Bragg gratings are provided to reflect all the traffic signal wavelengths for onward transmission along the trunk. In this case, the third port of the first circulator is connected to the trunk output and third port of the second circulator is connected to the branch drop output. In this case, all wavelengths will be amplified, the traffic signal remaining on the trunk being amplified twice and the add and the drop signals being amplified once.
Although these arrangements are useful, it is particularly desirable to simplify the ADM design to minimise the number of optical components, while obtaining low losses and a controllable and balanced system response. A particular difficulty arises when there are a plurality of fibers on the trunk. It is particularly desirable to handle effectively the adding or dropping of signals from a branch station to a trunk with such a plurality of fibers.
Accordingly, an earlier application of the present applicants, International Patent Application No. PCT/GB96/01891, discloses a plurality of arrangements in which an ADM combines traffic signals dropped from a plurality of trunk fibers onto a single spur drop fiber and splits from a single spur add fiber a plurality of add signals for adding to different fibers of the trunk. Such arrangements allow for mininisation of the number of optical components required and for effective balancing of losses. However, these designs do not themselves provide for efficient amplification of the traffic signals.
Accordingly, the invention provides an add/drop multiplexer for use in a wavelength division multiplexed optical network, the add/drop multiplexer having a first trunk input for receiving traffic signals from a first part of a first trunk fiber, a second trunk input for receiving traffic signals from a first part of a second trunk fiber, a first trunk output for outputting traffic signals to a second part of the first trunk fiber, a second trunk output for outputting traffic signals to a second part of the second trunk fiber, a branch input for receiving traffic signals from a branch input fiber, and a branch output for outputting traffic signals to a branch output fiber;
the add/drop multiplexer comprising:
means of routing from the first trunk input to the branch output a first
set of traffic signals at carrier wavelengths predetermined for transmission of signals from the first trunk fiber to a branch station and for routing from the second trunk input to the branch output a second set of traffic signals at carrier wavelengths predetermined for transmission of signals from the second trunk fiber to the branch station, and means for combining said first and second sets of traffic signals for output at the branch output;
means for separating traffic signals received at the branch input into a third set of traffic signals at carrier wavelengths predetermined for transmission of signals from the branch station to the first trunk fiber and a fourth set of traffic signals at carrier wavelengths predetermined for transmission of signals from the branch station to the second trunk fiber, and means for routing the third set of traffic signals to the first trunk output and the fourth set of traffic signals to the second trunk output respectively; and
one or more amplifiers located on traffic signal paths determined by said routing means, such that said one or more amplifiers amplify each routing for traffic signals determined by said routing means and each of said one or more amplifiers is adapted to amplify traffic signals for a plurality of said routings.
Preferably, the amplifiers are bidirectional, in which case at least one of the amplifiers can support traffic signal routings which are arranged to pass through that amplifier in opposite directions. Particularly appropriate are doped fiber amplifiers, in which case the add/drop multiplexer also comprises a pumping section for pumping the doped fiber amplifier. The pumping section comprise a first laser and a fiber optic coupler, wherein the output of the first laser is split into two output paths by the fiber optic coupler. A further advantageous arrangement provides redundancy by the addition of a second laser connected to the same side of the fiber optic coupler as the first laser so that light provided on each of these two output paths is a combination of light provided by the first laser and light provided by the second.
In preferred arrangements, the add/drop multiplexer comprises a plurality of optical circulators and a plurality of notch reflector filters, preferably fiber Bragg gratings. In preferred arrangements, the plurality of optical circulators forms a linear chain, and the first and last circulators of the chain are three port circulators. It is advantageous that a first set of notch reflection filters is then provided between the first and second circulator to reflect signals at a first set of one or more traffic signal wavelengths and a second set of notch reflection filters is provided between the penultimate and the last circulator to reflect signals at a second set of one or more traffic signal wavelengths. In a particularly preferred arrangement, at least two amplifiers are provided, wherein one of the two amplifiers is located between the first circulator and the first set of notch reflection filters and the other of two amplifiers is located between the second set of reflection filters and the last circulator. Advantageously in this arrangement, the pumping section has at least two outputs, one of the two outputs is coupled to the linear chain adjacent to these two amplifiers and the second of the two outputs is coupled to the linear chain adjacent to the second of the two amplifiers.
In a further aspect, the invention provides an add/drop multiplexer for use in a wavelength division multiplexed optical network, the add/drop multiplexer having a plurality of inputs each for receiving traffic signals from a different fiber of the network and a plurality of outputs each for outputting traffic signals to a different fiber of the network, wherein the add/drop multiplexer comprises a plurality of optical circulators and a plurality of notch reflection filters wherein:
the plurality of optical circulators form a linear chain;
the first and last circulators in the chain are three port circulators, wherein the first port of each of the first and last circulators is connected to a different input of the add/drop multiplexer, and wherein the third port of each of the first and last circulators is connected to a different output of the add/drop multiplexer, wherein the second port of each of the first and last circulators is connected to the adjacent circulator in the linear chain; and one or more further circulators in the linear chain are four port circulators, wherein a first port of each of said further circulators is connected to a different input of the add/drop multiplexer, a fourth port of each of said further circulators is connected to a different output of the add/drop multiplexer, and the second and third ports of each of said further circulators are connected to adjacent circulators in said linear chain.